1. Field of the Invention
The invention relates to transceivers for use in a communications system and, more particularly, to dual frequency phase locked loops for enabling transceivers to operate in multiple modes.
2. Description of the Related Art
In modern communications systems, multiple communications protocols may be used. A variety of protocols arise where each protocol supports a different level of communication service, or where technology improvements have facilitated the development of advanced protocols, but legacy protocols must simultaneously be supported. One example of a communication system that supports multiple protocols is a wireless local area network (LAN). A wireless LAN generally supports the various protocols under IEEE 802.11. The protocols are generally identified by a letter designation such as 802.11a, 802.11b, and 802.11g. When a transceiver uses multiple protocol modes, the multiple designations are listed. For example, a transceiver that uses all three 802.11 protocols is referred to as an 802.11a/b/g transceiver.
When developing a combined 802.11a/big transceiver, the choice between zero intermediate frequency (ZIF) and low intermediate frequency (LIF) in the transmit mode (TX) and receive mode (RX) paths is particularly important. The specification of the transmit mask, combined with the limited achievable image rejection, make ZIF the logical choice for use in the transmit mode. In the receive mode however; the choice is less straightforward. In the 802.11b protocol, the possibility of strong adjacent channel interference combined with a limited achievable image rejection more or less dictate ZIF as the appropriate frequency to be used for the receive mode. On the other hand, in the 802.11a/g protocols, the maximum adjacent channel interference is less strong, making LIF attractive in the receive mode because LIF avoids the DC offset problem.
The DC offset in a ZIF receiver has a number of sources including transmitter local oscillator frequency leakage, the receiver local oscillator feed through, and a receiver DC offset in the analog base band. To insure optimal dynamic range at the analog-to-digital converter, a slow varying DC offset can be removed by calibration in the analog domain, while a fast varying DC offset can be removed on a packet-by-packet basis in the digital signal processor.
The removal of the DC offset in a ZIF-based receiver is complicated by the frequency offset between the transmitter and receiver. The transmitter and receiver frequency offset is caused by the limited accuracy of the synthesizer crystal in the transmitter and receiver circuitry. The accuracy is generally ±20 ppm or ±40 ppm for the transmitter and receiver combined. Such inaccuracy requires frequency correction in the digital signal processor to ensure correct positioning of the Orthogonal Frequency Division Multiplexed (OFDM) carriers to avoid signal smearing. In a ZIF receiver, the necessary frequency correction in the 802.11 a/g modes interferes with the DC removal in the digital signal processor. While the DC offset due to the transmitter local oscillator leakage will appear as DC, the receiver DC offset shifts by an amount equal to the transmitter and receiver frequency offset after the frequency correction. This causes possible interference with the first OFDM sub-carrier at 312.5 kHz. Thus, LIF receivers are attractive when using 802.11 a/g protocols.
Summarizing, to produce a multi-mode 802.11 a/b/g transceiver, ZIF transmitter/ZIF receiver is a good choice for operating in 802.11b mode and ZIF transmitter/LIF receiver is a good choice for operating in 802.11a/g modes. However, this combination of receivers and transmitters is rarely used in practice, due to the limited time available to switch local oscillator frequency to accommodate ZIF receive b mode and LIF receive g mode. Switching between ZIF and LIF must occur during a very short period in which the following also occurs the preamble time, burst detection, channel estimation, AGC setting, antenna diversity switching and frequency offset and symbol timing correction. This only leaves a few microseconds to accurately switch the local oscillator frequency from LIF to ZIF modes, or vice versa. A conventional integer-N phase locked loop is unable to switch the local oscillator fast enough to accommodate both ZIF and LIF modes in a transceiver.
Therefore, there is a need in the art for a fast switching, dual frequency phase locked loop to facilitate the use of multiple operational modes and/or protocols in a transceiver.